Here in the 1852 LDS edition, the demonstrative this was accidentally set as the. For the second printing of that edition, also in 1852, the correct this was restored, probably by reference to the 1840 edition (although any earlier edition would have given the correct reading). Perhaps one could have figured out that this was correct without referring to any other edition. The critical text will maintain the demonstrative this here in Moroni 9:20.
Summary: Maintain in Moroni 9:20 the determiner this in the noun phrase “this horrible scene”, the reading of the earliest text.